The roof of a home protects it from Mother Nature's wrath. Rain, snow, wind, even sunlight can rapidly break down the interior components of a home. Washington State has all of these conditions and has specific building codes for roofing to prevent Mother Nature from damaging the interior of a home.
The roof underlayment is the waterproofing layer of the roof. The correct underlayment is 30 lb asphalt-based tar paper as required, at minimum, by code to be laid on top of the roof sheathing and overlap a minimum of 2 inches at the seams. The underlayment should project up and sidewalls where the roof meets the side of the house 12 inches. This prevents water infiltration along the edge of the roof. This code is required for pitched roofs of over a 2 pitch.
Flat roofs may or may not be required to have underlayment depending on the type of roof. For example, asphalt based roll roofing requires 30 lb asphalt-based underlayment for the added water protection. Rubber membrane roofing should not have an underlayment as the rubber is adhered to the roof surface for proper installation.
Roof coverings such as shingle, tiles, rubber, plastic or metal, must be rated as a class A fire retardant material. This is a requirement for homeowners insurance in Washington state and has been built into the local building codes to prevent the use of inferior roof covering products and to help facilitate updating homes from the old roofing products to the new roofing products.
Flashings must be galvanized steel, aluminum, or copper in the roof valleys, around chimneys and other penetrations. Rubberized flashing boots are acceptable around plumbing vents and electrical masts. Ice and water shield membrane is acceptable for valley flashing as well and is recommended along the eaves of the roof. However, the installation of ice and water shield along the eaves is not a requirement.
Roofing systems intended for ventilation should have a minimum of 1 square foot of ventilation for every 600 square feet of attic space. Attic insulation systems that are insulated to the roof sheathing, should have no ventilation and be sealed at the eaves.